Hard-disk drives (hereinafter referred to as HDDs) have been put to practical use as an information recording medium for recording massive amounts of information used with computers, etc. This HDD has a plurality of magnetic recording disks (hereinafter referred to as disks), and information recording to the disk or information reading from the disk is performed by positioning a reproduction/recording head (hereinafter referred to as a magnetic head) over a desired tack on the disk.
In this HDD, a rotary actuator using a voice coil motor (VCM) has been widely used to position the magnetic head over a predetermined position on the disk, because the actuator is light-weight, high in rotation speed and low in cost. This rotary actuator has its axis of rotation outside the disk, and rotates the magnetic head on the fore end thereof and positions it over a predetermined position on the disk. A servo method is a sector servo method in which a pattern, a cylinder number, and a sector number needed for positioning control are recorded every sector of each disk surface. The sector servo method has been widely used in small HDDs.
HDD capacity has enlarged rapidly as processes in computers and the like have advanced. To promote increased HDD capacity, the reproduction/recording head must be small. The applicant of this application uses an MR head as a magnetic head for recording and reproducing information.
This MR head consists of an MR element with an MR effect as a reproduction element or write element, and an inductive thin-film head as a recording element. The reproduction element and the recording element are formed independently. As a matter of common knowledge, the MR element has a magnetoresistive effect in which a resistance value changes by a magnetic field or magnetization, and can detect a magnetic field near the MR element. When detecting a magnetic field by a change in a resistance value, the MR element requires a bias magnetic field to enhance its sensitivity and its linearity to a varying magnetic field. A shunt bias or a soft bias has been used as a general method for obtaining this bias magnetic field.
In the shunt bias, a conducting wire (i.e., one wire) is disposed parallel to the MR element, and a magnetic field produced by passing a current through this wire is applied to the MR element. That is, the shunt bias applies a bias magnetic field to an MR element vertical to the disk surface. Further, in the soft bias, a soft magnetic film is disposed parallel to a MR element, the soft magnetic film is magnetized by a magnetic field produced when a bias current for reproduction is applied to the MR element, and a bias magnetic field is applied to the MR element by the magnetic field of the soft magnetic film produced by that magnetization.
In a HDD, the MR element and the recording element do not always pass over the same track position. If, as shown in FIG. 11(A), it is designed that a rotary actuator (not shown) is rotated and an MR element 110 and a recording element 120 are positioned over nearly the same track at a predetermined position on the disk (e.g., near the innermost circumference of the disk), the region of reproducible track width T.sub.R will be contained in the region of a recordable track width T.sub.W, because the recording element 120 is generally wider than the MR element 110. If, however, the rotary actuator is rotated toward the outer circumference of the disk, the MR element 110 and the recording element 120 will not pass over the same track, as shown in FIG. 11(B), because of a difference in skew angle and in position between the MR element 110 and the recording element 120 caused by rotation. That is, the region of the reproducible track width T.sub.R overlaps with only a portion of the region of the recordable track width T.sub.W and has a region that is not contained in the recordable track width T.sub.W. Therefore, in the case that, as shown in FIG. 11(B), the MR element 110 and the recording element 120 do not pass over the same track, a servo control (microjogging) is required which changes the head position reproduction and recording.
The aforementioned sector servo method, however, has disadvantages when the sector number is reproduced. For example, if, as shown in FIG. 12, a cylinder number, a position control pattern, and a sector number and data recorded at the time of format and thereafter are contained in sectors, the sector number must be rewritten to stop using defective sectors and reproduced immediately before information reproduction or recording without discriminating them.
If information is recorded when, as shown in FIG. 13(A), a sector number was recorded on the same track as data and a sector region SC and a data region D are formed on the same track, the recording element 120 is controlled to be positioned over the track, so the MR element 110 is moved off-track to the sector region SC, and the sector number immediately before data is reproduced. However, when information is reproduced, the MR element 110 is controlled to be positioned over the track as shown by broken lines in FIG. 13(A). Therefore, the MR element 110 can be moved to the sector region SC without being off the track, and the sector number and data can be reproduced. Thus, the MR element 110 is greatly off from the track at reproduction of the sector number during information recording.
In addition, if, as shown in FIG. 13(B), the sector region SC and data region D were recorded so that the sector region SC is axially aligned with the MR element 110 and the data region D is axially aligned with the recording element 120, there is no problem in information recording. However, since the MR element 110 is positioned over the track, that is, over the data region D (as shown by broken lines in FIG. 13(B)) at information reproduction, the MR element 110 is moved to the sector region SC off-track and the sector number is read off-track.
To overcome the problem above, a constitution is proposed such that, as shown in FIG. 13(C), the sector region SC is recorded so that it is off the recording element 120 and the MR element 110 by an equal quantity and such that, even most off-track, the sector number can be reproduced half off-track (FIG. 13(A)) where the sector number was recorded on the same track as data.
As described above, if an MR head comprising recording and reproduction elements separate from each other is used in an HDD of the rotary actuator and sector servo type, the sector number must be reproduced off-track equivalent to a displacement of about 0.2 to 1.0 .mu.m as an experimental value. In some cases, the sector number is not detected at all or is detected by mistake.
When the MR head is used in HDDs other than the servo sector type above, the track position during recording and track position during reproduction are also different because of a difference in skew angle and in position between the MR element 110 and the recording element 120 caused by the rotation of the MR head. Therefore, in some cases, the sector number is not detected or is detected by mistake, as in the case of the servo sector type.
In consideration of the facts above, an object of the present invention is to provide a servo method and a hard-disk system for an MR head which can reproduce information, making it possible to reduce cases in which information is not detected or is detected by mistake.